fix(strings): non fatal errors for [NULL] safety in vim_snprintf

context: vim_snprintf provides a "portable" replacement for a system
vsnprintf(). However we rely on system vsnprintf() in places. This
is quite arbitrary. I want to use system vsnprintf() only, and
reduce the machinery in string.c for typval printf only, which will
delet a lot of duplicated code.

The biggest hurdle here is not "portably" (we only support sane c
runtimes) but the following discrepancy: standard libc printf()
considers NULL args to %s to be undefined behavior strictly
while vim_snprintf() _defacto_ allows this by replacement to "[NULL]"

IMO, this should be seen as "graceful" error handling (nicer than crashing
on the user), not a valid means to have the string "[NULL]" intentionally
be shoved in the users face. more robust code should explicitly
check for NULL and replace it with a _appropriate_ fallback for
the situation, depending on what a NULL string actually means in the
context (unnamed buffer? anonymous namespace? global augroup?)

soo, this PR provides the most _gently_ incremental nudge towards
considering this case again as an error one could possible imagine.
If sanitizers are complied in, this minor incursion gets printed
into the sanitizers' log as a "report-and-continue" error with a
traceback. This doesn't annoy the end user or "stop the world"
in the test suite, but the CI will see the reported error and
fail the build, just like other non-fatal CI errors.

1 files changed, 5 insertions, 12 deletions

diff --git a/src/nvim/lua/executor.c b/src/nvim/lua/executor.c
index 5ac5e7203b..29c31a4c82 100644
--- a/src/nvim/lua/executor.c
+++ b/src/nvim/lua/executor.c
@@ -1131,12 +1131,6 @@ static void nlua_print_event(void **argv)
 static int nlua_print(lua_State *const lstate)
   FUNC_ATTR_NONNULL_ALL
 {
-#define PRINT_ERROR(msg) \
-  do { \
-    errmsg = msg; \
-    errmsg_len = sizeof(msg) - 1; \
-    goto nlua_print_error; \
-  } while (0)
   const int nargs = lua_gettop(lstate);
   lua_getglobal(lstate, "tostring");
   const char *errmsg = NULL;
@@ -1148,22 +1142,21 @@ static int nlua_print(lua_State *const lstate)
     lua_pushvalue(lstate, -1);  // tostring
     lua_pushvalue(lstate, curargidx);  // arg
     // Do not use nlua_pcall here to avoid duplicate stack trace information
-    if (lua_pcall(lstate, 1, 1, 0)) {
-      errmsg = lua_tolstring(lstate, -1, &errmsg_len);
+    if (lua_pcall(lstate, 1, 1, 0) || lua_type(lstate, -1) != LUA_TSTRING) {
+      // NB: this might try tostring once more to print a weird error from
+      // a __tostring methamethod but we stop after that.
+      errmsg = nlua_get_error(lstate, &errmsg_len);
       goto nlua_print_error;
     }
+
     size_t len;
     const char *const s = lua_tolstring(lstate, -1, &len);
-    if (s == NULL) {
-      PRINT_ERROR("<Unknown error: lua_tolstring returned NULL for tostring result>");
-    }
     ga_concat_len(&msg_ga, s, len);
     if (curargidx < nargs) {
       ga_append(&msg_ga, ' ');
     }
     lua_pop(lstate, 1);
   }
-#undef PRINT_ERROR
   ga_append(&msg_ga, NUL);
 
   lua_getfield(lstate, LUA_REGISTRYINDEX, "nvim.thread");